At the heart of Christianity is the doctrine of the mystical union of the believer with Christ. The New Testament does not only call us to believe in Christ, but to believe into Christ. Faith links us directly into Christ. We become in Him and He in us. This mysterious union is carried over into the relationship between Christ and the church. The church is His bride, whom He has brought into a real, profound, and powerful union.

We normally assume that the image of the church as the bride of Christ is a metaphor borrowed from the institution of human marriage. In this case, the earthly serves as the model for the heavenly. Perhaps that is the intent of Scripture. I am not sure. It may well be the other way around. It may be that the earthly estate of marriage is based on the heavenly model of the mystical union of Christ and His bride. Marriage is the reflection of the heavenly reality, not the basis for a heavenly image.

The perfect unity of persons existed in eternity in the nature of the Trinity itself. Though the Father, Son, and Holy Spirit are not "one flesh," they are one being in perfect eternal harmony. In the Godhead, there is no possibility of divorce. It is a union that cannot and will not be broken. It is the eternal pattern of relationship that defines our human relationship. We share with God not only an analogy of being, but also an analogy of relationship. It is found in the mystery of marriage.

http://www.ligonier.org/learn/devotionals/defining-our-relationship/